valley floor becomes a ridge bounded by newly formed valleys on each side". In the case of Forty Hill, the permeable Boyn Hill Gravel on the hill protected the otherwise easily eroded London Clay underneath from being removed, while nearby watercourses cut down into exposed London Clay). The brooks approached each other quite closely either side of Beggars Hollow (close to where, today, the Rose and Crown public house is located on Clay Hill). The dividing line between them was thus lowered by erosion at that point. During further cold climate periods, the Lea laid down Taplow Gravel, east and south of Forty Hill. That was followed by Brickearth, a partially wind-blown deposit. Later, the Lea moved further east to its present line, well away from Forty Hill, cutting down to a today's altitude of c20m at Waltham Abbey. The low point between Turkey Brook and Cuffley Brook at Beggars Hollow was breached, probably in the relatively recent geological past. The Turkey Brook thus changed its course, there to go north-east, through a water gap (that is, a gap which, in this case, has been "formed through...erosion of two streams on opposite sides of a ridge, ultimately resulting in the capture of one stream by the other"). As a result, the Turkey Brook joined the Cuffley Brook, in Whitewebbs Park, thus defining the western edge of Forty Hill and isolating it from higher ground to the west. East of the junction of Turkey Brook and Cuffley Brook, the stream is now known as Turkey Brook - and also locally as Maidens Brook. The brooks subsequently cut down a little further and alluvium was deposited along the valley bottoms in Hilly Fields, Whitewebbs Park and the grounds of Forty Hall. A dry, former stream channel at c30m was left, running just north of Clay Hill, from Beggars Hollow to a point close to today's junction of Clay Hill and Baker Street. That channel now defines the southern boundary of Forty Hill. When the New River was built, it followed the 30m contour from Hertfordshire south towards London, down the Lea valley. But, in Enfield, the engineers who constructed it took the New River on a loop going west, to the north of Forty Hill, and then across Cuffley Brook near Flash Lane (and, later, across an aqueduct there). From that point, they took it south-east, through the water gap at Beggars